Coming of Age Trailer Photo Effect

I'm trying to make a movie using iMovie and really like an effect that is done in the Coming of Age Trailer. I was wondering if anyone can give simple instructions on how the effect was created in the trailer.

Near the beginning of the Coming of Age Trailer, after the "story of a boy" transition, there are a few photo shots where the photo is embedded in a semi-diagonal frame/border. How is this effect done? I tried to convert the trailer into a movie but looking through the project pane, I still can't figure out how it's done. Thank you!